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Computed Tomography (CT or CAT) Scan


Computed Tomography (CT or CAT) Scan

What is a CT or CAT scan?

Photo of patient being perpared for CT scan

A CT or CAT scan is a diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce horizontal, or axial, images (often called slices) of the body. A CT scan shows detailed images of any part of the body, including the bones, muscles, fat, organs, and blood vessels. CT scans are more detailed than standard X-rays.

In standard X-rays, a beam of energy is aimed at the body part being studied. A plate behind the body part captures the variations of the energy beam after it passes through skin, bone, muscle, and other tissue. While much information can be obtained from a regular X-ray, a lot of detail about internal organs and other structures is not available.

In computed tomography, the X-ray beam moves in a circle around the body. This allows many different views of the same organ or structure, and provides much greater detail. The X-ray information is sent to a computer that interprets the X-ray data and displays it in two-dimensional form on a monitor. Newer technology and computer software makes three-dimensional (3-D) images possible.

CT scans may be done with or without contrast. "Contrast" refers to a substance taken by mouth or injected into an intravenous (IV) line that causes the particular organ or tissue under study to be seen more clearly. Contrast examinations may require you to fast for a certain period of time before the procedure. Your doctor will notify you of this prior to the procedure.

CT scans may be performed to help diagnose tumors, investigate internal bleeding, or check for other internal injuries or damage.

You may want to ask your doctor about the amount of radiation used during the CT procedure and the risks related to your particular situation. It is a good idea to keep a record of your past history of radiation exposure, such as previous CT scans and other types of X-rays, so that you can inform your doctor. Risks associated with radiation exposure may be related to the cumulative number of X-ray examinations and/or treatments over a long period of time. If you are pregnant or suspect that you may be pregnant, you should notify your doctor.

Advances in computed tomography technology include the following:

  • High-resolution computed tomography. This type of CT scan uses very thin slices (less than one-tenth of an inch), which are effective in providing greater detail in certain conditions such as lung disease.

  • Helical or spiral computed tomography. During this type of CT scan, both the patient and the X-ray beam move continuously, with the X-ray beam circling the patient. The images are obtained much more quickly than with standard CT scans. The resulting images have greater resolution and contrast, thus providing more detailed information. Multidetector row helical CT scanners may be used to obtain information about calcium build-up inside the coronary arteries of the heart.

  • Ultrafast computed tomography (also called electron beam computed tomography). This type of CT scan produces images very rapidly, thus creating a type of "movie" of moving parts of the body, such as the chambers and valves of the heart. This scan may also be used to obtain information about calcium build-up inside the coronary arteries of the heart, but the helical scanners are much more common.

  • Computed tomographic angiography (CTA). Angiography (or arteriography) is an X-ray image of the blood vessels. A CT angiogram uses CT technology rather than standard X-rays or fluoroscopy to obtain images of blood vessels, for example, the coronary arteries of the heart.

  • Combined computed tomography and positron emission tomography (PET/CT). The combination of computed tomography and positron emission tomography technologies into a single machine is referred to as PET/CT. PET/CT combines the ability of CT to provide detailed anatomy with the ability of PET to show cell function and metabolism to offer greater accuracy in the diagnosis and treatment of certain types of diseases, particularly cancer. PET/CT may also be used to evaluate epilepsy, Alzheimer's disease, and coronary artery disease.

Studies show that 85 percent of the population will not experience an adverse reaction from iodinated contrast; however, you will need to let your doctor know if you have ever had a reaction to any contrast dye, and/or any kidney problems. A reported seafood allergy is not considered to be a contraindication for iodinated contrast. If you have any medical conditions or recent illnesses, inform your doctor. The effects of kidney disease and contrast agents have attracted increased attention over the last decade, as patients with kidney disease are more prone to kidney damage after contrast exposure. If you are pregnant or think you may be pregnant, you should notify your health care provider. If you are claustrophobic or tend to become anxious easily, tell your doctor ahead of time, as he or she may prescribe a mild sedative for you before the procedure to make you more comfortable. It will be necessary for you to remain still and quiet during the procedure, which may last 10 to 20 minutes.

How is a CT or CAT scan performed?

CT scans can be performed on an outpatient basis, unless they are part of a patient's inpatient care. Although each facility may have specific protocols in place, generally, CT scans follow this process:

  1. When the patient arrives for the CT scan, he or she will be asked to remove any clothing, jewelry, or other objects that may interfere with the scan.

  2. If the patient will be having a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast medication. For oral contrast, the patient will be given the contrast material to swallow.

  3. The patient will lie on a scan table that slides into a large, circular opening of the scanning machine.

  4. The CT staff will be in another room where the scanner controls are located. However, the patient will be in constant sight of the staff through a window. Speakers inside the scanner will enable the staff to communicate with and hear the patient. The patient may have a call bell so that he or she can let the staff know if he or she has any problems during the procedure.

  5. As the scanner rotates around the patient, X-rays will pass through the body for short amounts of time. The motion is hidden inside the gantry, the doughnut-shaped part of the machine. The patient may hear buzzing, whirring, and clicking as the X-ray tube rotates.

  6. The X-rays absorbed by the body's tissues will be detected by the scanner and transmitted to the computer.

  7. The computer will transform the information into an image to be interpreted by the radiologist.

  8. It is very important that the patient remain very still during the procedure. You may be asked to hold your breath at various times during the procedure.

  9. The technologist will be watching the patient at all times and will be in constant communication.

  10. The patient may be asked to wait for a short period of time while the radiologist examines the scans to make sure they are clear. If the scans are not clear enough to obtain adequate information, the patient may need to have additional scans performed.